Fluid catalytic cracking (FCC) is carried out by contacting hydrocarbons in a tubular reaction section or riser with a catalyst made up of a fine particulate material. The most common feedstocks to be submitted to a FCC process are usually those refinery streams from vacuum tower side cuts named heavy vacuum gasoils (HVGO) or heavier than the latter, from the bottom of atmospheric towers, named atmospheric residua (ATR), or still, admixtures of these streams.
These streams, having densities typically in the range of from 8° to 28° API, in order to deeply alter their composition and convert them to lighter, more valuable hydrocarbon streams, should be submitted to a chemical process such as the catalytic cracking process.
During the cracking reaction, substantial amounts of coke, as a reaction by-product, are deposited on the catalyst. Coke is a high molecular stock made up of hydrocarbons that contain of from 4 wt % to 9 wt % hydrogen in their composition.
The coke-recovered catalyst normally designed as “spent catalyst” is directed to the regenerator. In the regeneration zone, in a regenerator vessel kept at high temperature, coke deposited on the surface and in the catalyst pores is burned. Coke withdrawal by combustion leads to the catalyst activity recovery and releases heat in a sufficient amount to provide for the thermal requirement of the catalytic cracking reactions.
The fluidization of the catalyst particles by gaseous streams allows the catalyst transport between the reaction zone and the regeneration zone and vice-versa. The catalyst, besides doing its essential task of promoting the chemical reaction catalysis, is also the heat transport medium from the regenerator to the reaction zone.
The technique is abundant in descriptions of hydrocarbon cracking processes in a stream of fluidized catalyst, with catalyst transport between the reaction zone and the regeneration zone, and burning of coke in the regenerator.
In spite of the rather long existence of FCC processes, there is a continuous search for new techniques for improving the process, increasing the yield in more valuable products, such as gasoline and LPG. Broadly, it may be stated that the main objective of FCC processes is the maximization of said more valuable products.
The maximization of these products is basically obtained in two ways. One is the increase of the so-called “conversion”, corresponding to the reduction in the production of heavy products such as clarified oil and light cycle oil. Another way is the reduction in the coke and fuel oil yields, that is, through the lower “selectivity” to these products.
The lower production of these two latter products, increasing the process selectivity to the target products, has as further beneficial results the need of smaller air blowers and wet gas compressors, those being big-sized, energy-consuming machines generally limiting of the UFCC capacity. Besides, it is economically interesting to promote the rise of more valuable products such as gasoline and LPG.
One important aspect to consider is the interest or need to increase LPG production according to the refiner's needs.
The experts know that an important feature of the FCC process is the initial contact of the catalyst and feed, this having a paramount influence on the conversion and selectivity of the process to generate valuable products. In a FCC process, the pre-heated hydrocarbon feed is injected near the bottom of a conversion zone or riser, where it contacts the flux of regenerated catalyst. It is from the regenerated catalyst that the feed receives heat in sufficient amount to vaporize and provide for the thermal demand of the endothermic reactions that predominate in the process.
After the riser, a long vertical tube having dimensions in an industrial unit of ca. 0.5 m to 2.0 m diameter by 25 m to 40 m height, where the chemical reactions occur, the spent catalyst, having coke deposited on its surface and pores, is separated from the reaction products. The spent catalyst is then directed to the regenerator to burn the coke in order to have its activity restored and generate the heat that, being transferred from the catalyst to the riser, will be used by the process.
The conditions existing in the feed injection location of the riser are determining as related to the products formed in the reaction. In this region the initial mixture of the feed and regenerated catalyst occurs, heating the feed until the boiling point of its constituents is attained with the vaporization of most of such constituents. The total residence time of the hydrocarbons in the riser is around 2 seconds.
In order to process the catalytic cracking reactions, it is required that the feed vaporization in the region of admixture with the catalyst occurs quickly so that the vaporized hydrocarbon molecules may contact the catalyst particles—the size of which is around 60 microns—permeating through the catalyst micropores and reacting in the acidic sites. Failure in achieving this quick vaporization results in the thermal cracking of the feed liquid fractions.
It is well known that thermal cracking favors the build up of by-products such as coke and fuel gas, mainly during the cracking of residual feeds. Coke poisons the acidic sites and may even block catalyst pores. Therefore, thermal cracking in the riser bottom undesirably competes with the catalytic cracking, object of the process.
The optimization of the feed conversion usually requires the maximum coke removal from the catalyst in the regenerator. Coke combustion may be obtained in a partial or total combustion regimen.
In the partial combustion regimen, the gases produced by coke combustion are mainly made up of CO2, CO and H2O and the coke content in the regenerated catalyst is of the order of 0.1 wt % to 0.3 wt %. In the total combustion regimen, to be carried out in the presence of larger oxygen excess, practically all the CO produced in the reaction is converted to CO2.
The oxidation reaction of CO to CO2 is highly exothermic, making total combustion to occur with a large heat release, resulting in high regeneration temperatures. However, total combustion leads to a catalyst having less than 0.1 wt % and preferably, less than 0.05 wt % coke, this being a favorable feature relative to the partial combustion, besides avoiding the need of a costly boiler for further CO combustion.
The coke increase on the spent catalyst causes an increase of the coke burned in the regenerator by mass unit of the circulated catalyst. In conventional FCC units heat is removed from the regenerator in the combustion gas and mainly in the hot regenerated catalyst stream. An increase in the coke content on the spent catalyst increases the temperature of the regenerated catalyst as well as the temperature difference between the regenerator and the reactor.
Therefore a decrease in the regenerated catalyst flow rate to the reactor, normally designated as catalyst circulation rate, is required in order to attend to the reactor thermal demand and keep the same reaction temperature. However, the lower catalyst circulation rate required by the larger temperature difference between the regenerator and the reactor leads to a lower catalyst/oil ratio, this in turn reducing conversion.
Thus, catalyst circulation from the regenerator to the reactor is ascertained by the riser thermal demand as well as by the regenerator temperature, which is a function of coke production. Since the catalyst circulation itself affects coke produced in the riser, it is concluded that the catalytic cracking process works under a thermal balance regimen. In view of the preceding, operation at high regeneration temperatures is to be avoided.
Generally, on using modern FCC catalysts, regenerator temperatures and therefore regenerated catalyst temperatures are kept below 760° C., preferably below 732° C., since activity loss would be severe above this figure. A desirable operation range is of from 685° C. to 710° C. The lower limit is dictated mainly by the need to secure suitable coke combustion.
On processing increasingly heavy feeds, there is a tendency to increase coke production and the operation under total combustion requires the use of catalyst coolers to keep the regenerator temperature within acceptable limits. Generally, catalyst coolers remove heat from a regenerator catalyst stream, and return to said vessel a substantially cooled catalyst stream.
As for the fluid-dynamic features of the riser, where the catalytic cracking reactions of the invention occur, it is well known that catalyst solid particles are entrained in the reaction medium during contact with the feed and other vaporized materials.
This kind of reactor is normally of tubular shape where, in order to reduce by-products, operation should be carried out according to a hydrodynamic flow regimen, so that the superficial gas velocity is high enough to cause that catalyst flux is in the same direction as that of the feed and of other gases present therein. That is, the liquid and vaporized feed entrains the catalyst particles throughout the entire path in the tubular reactor.
These flow regimens are known by the experts as fast fluidized bed, riser regimen or more generally as transport regimen, those regimens being the preferred ones when one deals with reaction systems that require continuous flow reactors.
Generally, for a certain cross section area of a tubular reactor, which is a function of the reactor diameter, the catalyst concentration, in a fluidized bed reactor, is reduced as a result of increased superficial gas velocity. The higher the superficial gas velocity, the higher will be the reactor lengths required to allow that a certain amount of feed may contact the required amount of catalyst. Those higher superficial gas velocities require a higher L/D (Length/Diameter) ratio or aspect ratio of the reactor, which is the ratio between the reactor length and its diameter.
In the patent literature several publications suggest the multiple injection of the same feed in FCC units.
U.S. Pat. No. 3,246,960 teaches an FCC apparatus built so that the injection of the same feed in different locations of the riser is carried out so as to promote a more uniform mixture between feed and catalyst, with the consequent increase in gasoline octane rating.
International publication WO 0100750A1 teaches the re-cracking of naphtha to increase LPG yield, simultaneously with the split-feed injection of the same feed. The split-feed is injected in at least two different locations above the reactor lower position. The process aims at maximizing diesel oil production.
U.S. Pat. No. 4,869,807 teaches a process for converting a non-segregated hydrocarbon feed in a FCC reactor in the presence of a zeolitic catalyst for producing gasoline. The same feed is divided in portions and injected into a plurality of locations along the length of the FCC reactor, with of from 60 to 75% by volume being injected in the lowest injection position. The distance between this location and the immediately superior location comprises at least 20% of the total reactor length. Multiple injection would allow increased gasoline octane rating.
U.S. Pat. No. 5,616,237 teaches the same technique of multiple injection of the same feed in different locations to secure selectivity improvements. This approach reduces the contact time of the feed, with the consequent bottom conversion. It is also suggested to promote a recycle of the non converted friction to several injection locations along the riser length.
U.S. Pat. No. 6,416,656 discloses a process for catalytically cracking hydrocarbon stocks in a riser or fluidized bed reactor to increase simultaneously the yields of diesel and liquefied gas. The process includes the steps of: first, charging a gasoline stock and a catalytic cracking catalyst into a lower zone of the reactor to permit contact between the catalyst and the gasoline stock and to produce a liquefied gas-rich oil-gas mixture containing reacted catalyst. The resulting liquefied gas-rich oil-gas mixture (still containing reacted catalyst) is then introduced into a reaction zone above the lower zone of the reactor. Simultaneously, at least one conventional catalytic cracking hydrocarbon feed is also fed independently into at least two sites situated at different heights above the lower zone of the reactor. The resulting mixture is then separated in a conventional fashion.
Another approach from the patent literature involves injecting an auxiliary stream such as water or petroleum fractions in a location downstream of the injection of the feed to be cracked in order to promote an increase in the mixing temperature in the area of the feed injection. This is done aiming at increasing the vaporized percent of residual feeds, without altering the riser outlet temperature.
Such an approach is taught in U.S. Pat. No. 4,818,372 that relates to a FCC apparatus with temperature control including an upflow or downflow reactor, a device to introduce the hydrocarbon feed under pressure and in contact with a regenerated cracking catalyst. The FCC apparatus comprises further at least a device for injecting an auxiliary fluid downstream of the reactor zone where feed meets the catalyst, whereby it is desired to attain a higher temperature in the mixing zone of feed and catalyst. This document does not contemplate feed segregation, rather, it makes use of an inert, external fluid the main effect of which is the cooling of the injection region of said fluid, with temperature control and increase in catalyst circulation rate. In this respect please see Example 1, column 7, lines 55 to 60 of said patent, where it is defined that the feed is the same feed, injected once in the riser base while the other injection is effected with a cooling fluid as water or either a product of the cracking itself. The proposed process is directed to the cracking of a residual feed, the main feature of which is to contain at least 10% of a fraction having boiling point higher than 500° C. The desired goal when increasing the mixture temperature is to secure the vaporization of heavier fractions, while at the same time promoting a thermal shock on said fractions, aiming initially at converting the bigger molecules into lighter compounds, able to vaporize and catalytically cracking in a further step.
This is attained by injecting an auxiliary fluid above the feed injection location, from which the cracking reactions occur under milder conditions, at constant reaction temperature and independently of the desired mixing temperature.
The goal of the present invention is different and directed to the situation where feeds of different crackability are processed at the same time in one single riser. Under these conditions, it is suggested to inject the feed of lower crackability, the coke selectivity of which as well as the contaminant concentration is higher, in a riser downstream injection location. This aims at increasing the severity of the reactions of the feed of better quality injected in the beginning of the reactive section of the riser aiming mainly at higher LPG yields. This is obtained by a localized increase in the regenerated catalyst circulation as well as of the temperature of the riser section comprised between the two injections. Further, regenerated catalyst that contacts the better quality feed in the beginning of the riser reactive section is less deactivated by virtue of the local absence of contaminants as well as the higher coke production caused by the lower crackability feed.
The injection location of the feed of lower crackability in the riser is chosen so as to maximize LPG production, and is a function of the properties of the different feeds to be processed, as well as of the riser outlet reaction temperature.
A further distinguishing point between the present invention and U.S. Pat. No. 4,818,372 is that in this latter the total catalyst circulation is substantially increased. This may be observed from Example 1, in the Table of column 8, which sets forth an increase in the catalyst circulation rate from 4.6 to 6.7 by injecting a certain flow rate of water in the middle location of the riser. As a consequence, more coke will be formed, overloading the air blower of the regeneration section that normally is already very tight in terms of accepting any coke increase.
In the present invention, the resulting rise in catalyst circulation rate is only local, being limited to the section comprised between the lower and upper injections, but there is no significant increase in total catalyst circulation rate. Actually, as the lower crackability feed and normally having higher coke selectivity, is processed in the riser under milder temperature and contact time conditions, it is to be expected coke production to be slightly reduced.
Further, in the downstream injection location, the catalyst is recovered by a considerable content of deposited coke, this making it less selective to further coke formation. This way no overburden is expected on the air blower of the regeneration section, instead, a relief is to be expected.
A further disadvantage of the teachings of said U.S. Pat. No. 4,818,372 is the overburden of the riser, reactor cyclones, transfer line, main fractionator as well as of the top condensers of the fractioning section at the moment of the injection of make up water in the riser. This leads to adapt the dimensioning of most of the equipment to the requirement of the claimed process.
Besides, injecting water in the riser means a poor energetic balance of the FCC process, since all the energy that water removes from the converter is lost when the same water condenses on the top of the main fractionator coolers. It should also be mentioned the further disadvantage of additional acidic water generation in the refinery.
As taught in U.S. Pat. No. 4,818,372, the segregated injection of an external stream in a downstream riser location is carried out aiming at controlling the riser temperature profile. This makes possible to keep the upstream section of the riser at a relatively higher temperature without altering the riser top temperature or TRX (reaction temperature). Such control may also be carried out through a heavy naphtha recycle, as taught in U.S. Pat. No. 5,087,349.
Aiming at the same goal, U.S. Pat. No. 5,389,232 teaches a heavy naphtha recycle in downstream riser locations.
Aiming at minimizing naphtha overcracking reactions, U.S. Pat. No. 4,764,268 suggests the injection of a LCO stream in the top of the riser.
A similar alternative, taught in U.S. Pat. No. 5,954,942 aims at increasing conversion, through a quench or quick cooling with the aid of a steam auxiliary stream in the riser upper region.
International publication WO 93/22400 mentions the possibility of injecting along the riser a cracking product such as LCO aiming at cooling the riser and consequently promoting an increase in the catalyst circulation rate so as to make possible improved performance of ZSM-5 additives.
Contrary to U.S. Pat. Nos. 4,818,372, 4,764,268, 5,389,232, 5,954,942 and International publication WO 93/22400, in the present invention the feed injected in the one or more downstream riser locations is not an auxiliary external stream but rather one of the streams that normally make up the feed of the FCC unit. Since the segregated feed is injected at a temperature equal or higher than the feed temperature, the improved yields should not be considered as caused by an increase in the total catalyst circulation rate.
As regards the injection of the segregated feed in different locations of the riser, some publications suggest to differentiate feeds as a function of the nitrogen content only.
Thus, U.S. Pat. No. 4,985,133, aiming at reducing NOx release into the regenerator, teaches an alternative for the injection of the higher total nitrogen feed in the riser base, the less contaminated feed being injected in a higher nozzle.
U.S. Pat. No. 4,218,306 teaches a FCC process for producing gasoline and distillate by combining cracking of a distillation gasoil injected in the base of a cracking zone of a riser for admixture with a regenerated catalyst to form a catalyst suspension at high temperature. A second hydrocarbon fraction having more difficult cracking features is charged at a location 3.05 m to 9.14 m (10 to 30 feet) downstream the first injection. The riser outlet temperature is limited to the range between 482° C.-593° C. (900° F. to 1100° F.), preferably 510° C.-530° C. (950° F. to 985° F.).
Said U.S. Pat. No. 4,218,306 is directed to improved gasoline yields, as set forth in the main claim. In a patentably distinguishing way, the present invention is a much more flexible process, directed to either LPG only or to the sum LPG+gasoline, according to the injection location of the feeds in the riser as well as the desired riser outlet temperature. Besides, contrary to the teachings of said US patent, according to the invention, the injection of the lower crackability feed is not limited to the riser section placed 10 to 30 feet (corresponding to 6% to 30% of the reactive section of a typical industrial riser) downstream of the riser base injection of the better crackability feed.
In the present invention the injection location of the lower crackability feed is set forth aiming at obtaining the maximum possible LPG yield. Such location is a function of the properties of the feeds of different sources to be processed, of the percent of the lower crackability feed processed based on the total feed flow rate as well as of the riser outlet reaction temperature. Said injection location may be positioned at any location downstream the injection of the lower feed, but preferably of from 10% to 80% of the riser reactive section. As a general rule, the ideal location for injecting the lower crackability feed is that, which provides for the operation conditions favoring the maximization of LPG production in the section between the two feed injections. Further, said location should conform to the minimum residence time required by the lower crackability feed to undergo the desired conversion to lighter products, including LPG.
It should be noted that in column 4, line 3 of U.S. Pat. No. 4,218,306, it is stressed that the downstream injection should be submitted to very slight or no heating at all, this featuring a feed cooling or quenching, such cooling being completely absent from the inventive process. Therefore, the concept of the said US patent, as applied to the main objective of the present invention, that is, maximum LPG production would not lead to the desired results.
U.S. Pat. No. 6,123,832 teaches a FCC process for the conversion of hydrocarbon mixtures based on a non-linear phenomenon consisting in the fact that the lower yield in valuable products is not linearly reduced, neither the coke yield increases linearly, with the increase in heavy component in the FCC feed.
This means that the marginal deleterious effect caused by feed contaminants on the FCC catalyst is weaker with the increase in heavy components. Alpha and beta different quality feeds are to be injected in different nozzles. Alternatively, different nozzles may be used. Still alternatively, the riser is divided in two zones for separate cracking in one portion of the riser. Thus, the benefit of using at least one high CCR feed stems from the fact that the lower CCR feed increases conversion to a much higher degree than the conversion loss due to the higher CCR content feed.
The conditions for differentiating alpha- and beta-feeds are: a) the CCR figures differ from at least 2 points in wt %; or b) they differ in hydrogen content by at least 0.2 wt %; or c) they differ in API gravity by at least two points; or d) they differ in nitrogen content by at least 50 ppm; or e) they differ in the C/H ratio by at least 0.3; or f) they differ in average boiling point by at least 93.3° C. (200° F.). The technique taught in said US patent is not clear as regards which feed is to be injected in which nozzle or riser position, or in which riser. One claim is directed to the methodology for calculating possible feed mixtures that could lead to desirable results in terms of valuable products. Injection is non-linear (claim 2, column 9).
Another alternative is the injection of an external stream such as an alcohol, ether or a gasoil of better quality than the feed injected in the riser base, as taught in U.S. Pat. No. 5,271,826. This approach does not contemplate feed segregation according to the concept of the invention.
Another approach for feed segregation, as taught in U.S. Pat. No. 4,422,925 and U.S. Pat. No. 3,617,497 is based on the difference between feeds exclusively focused on molecular weight, while suggesting multiple injections in the riser. The lower molecular weight feed is injected in the riser base aiming at maximizing gasoline yields. However, as will be seen hereinafter in the present specification, a single parameter for differentiating feeds is not sufficient for obtaining the desired results in terms of yields and products.
On the other hand, it is well known that the density is closely associated to the extent of feed contamination, as reported on page 132 of the article by M. A. Torem et al., in “Development of a new coefficient to predict FCC feedstock cracking”, ACS 206th National Meeting—Advances in Fluid Catalytic Cracking—1993, Chicago, USA.
The considerations set forth above indicate that, in spite of the extended literature and patent publications, there is no description nor suggestion, In isolated or combined way, of a FCC process free of overall sensible cooling effect and without significant alteration of the total catalyst circulation rate, having improved conversion to light products such as LPG and gasoline, this being obtained from a mixed A and B hydrocarbon feed where feed B is produced by a thermal process or by physical separation, is more selective to coke formation relative to the feed to be injected in the base of the riser reactive section, is more refractory to cracking and is more heavily contaminated, where the conditions for injecting the segregated feed involve suitable distances between the injection locations in the riser and optimized dispersion of both feeds A and B aiming at maximizing LPG production, such process being described and claimed in the present application.